Electrical circuit



. J ly 16, 1 J. N. WHITAKER ELECTRICAL CIRCUIT Filed March 31. 1942 2 Sheets-Sheet l .141 v T112154. flu I AUflU I n n n fluflufl flv I m T1 21). E J' 1 T ME INVEN'TOR JAMES NAPOLEON WHITAKER v am ATTORNEY (mark-w J July 16, 1946. J. N. WHITAKER 2,404,307

ELECTRICAL CIRCUIT Filed March 31. 1942 2 Sheets-Sheet 2 u WuE Q2333 INVENTOR MES Mpomw Mun/rm.

ATTORNEY ba s .i lava .1 l

. I i 1 R @N Patented July 16, 1946- ELECTRICAL CIRCUIT James Napoleon Whitaker, West Englewood, N. J assignor to Radio Corporation of America, a

corporation of Delaware Application March 31, 1942, Serial No. 436,983

This invention relates to electrical circuits and, more particularly, to electrical circuits and methods for converting time-modulated impulses into amplitude variations.

In many applications in electrical signalling, it

is desirable to signal by means of time variation of a signalling current in which the current has the value either zero or a predetermined constant finite value and in which the information is transmitted by varying the time duration either 4 of the spaces between the pulses or the duration of the pulses themselves or a combination of variation of space and marking interval. At the same time it is desired to utilize the received energy in the form of amplitude variations. Forexample, in facsimile transmissionover. long distances, in order to overcome the efiects of fading it is desirable to use the constant frequencyivariable dot type oitransmission such as is exemplified by the Shore et 91. Patent 2,083,245. n the other hand, for recording the transmitted picture it is desired to have a continuous variation. between the signals representative of white to black so that the receivedpicture appears to have continuous tones rather than a screenpattemfsuch as the recording of CFVD dots give. --'The reason for this is that where the pictures transmitted and received are to be used in printed reproduction the picture is ordinarily screenediagain by the photoengraver. The double screening produces a moir efiect which is very disturbing to the observer. Moreover, this effect tends to reduce the amount of detail in the picture. Other examples will be immediately obvious,-as, for example, in measuring systems where it isdesi'red to have continuous recording'graphs rather than the broken line graph furnished bytime-modulated signals. i

Accordingly, by my invention, :1 provide a method and apparatus which will convert the time-modulated signals into amplitude-modulated signals or into signals .of a'unidirctional current whose amplitude varies with time. In accordance with my invention the incoming pulse signals are..utilized to derive a charge whose magnitude isv proportional to the timeduration of the pulse. The stored charge is thereafter utilized to control the amplitude of a continuous unidirectional current such that the amplitude of the current is proportional to the stored charge. Thereafter the stored charge is dissipated. During the time the stored charge is controlling the amplitude of the output current a secondimpulse is being used to produce a second j stored charge, and when the first stored charge is being dissipated, a third pulse is used to derive a stored charge while the second stored charge is used to control the amplitude of the output current. These cyclic steps are thereafter repeated. The output current therefore has a step by step from faulty operation.

12 Claims. (Cl. 250-) variation in amplitude, the magnitude of the amplitude being proportional to the time duration of the original pulses.

Further, in accordance with my method, I have provided apparatus for carrying outthe described method of operation, utilizing thermionic tubes and without requiring the aid of mechanical commutators. This is necessary in order that therequisite speed of operation can be obtained with a minimum of maintenanc and freedom Accordingly, it isthe main object-of my invention to provide a-new' ;method andapparatus for converting i time-modulated; I pu s s into amplitude-modulated currents: I

Another object .of myi invention lis to provide a method'of converting impulseswhosetimeduration varies into a continuous unidirectional current whose amplitude varies-continuously. 'As'till further object ,Qf any invention is to provide apparatus embodyin a thermionic commutator for convertingja: series;of;}pu1ses into a unilateral current'whosezmagnitude is proportional tothe time'duration-of the received pulses.

Another object of my invention is to provide apparatus suitable to produceimproved pictures from received CFVD signals. .w

Another object of 'my' invention will become apparent to those skilled in the artupon reading the following detailed description in connection with the'drawings.

In the drawings, I have shown in Figure 1 a schematic sketch embodying a mechanical commutator to illustrate the principles of my method arid apparatus, and have shown in Figures 2a, 2b, 20 representative graphs of the received currents and output-currents,-while in Fig. 3 I have shown in considerable detail an electrical circuit diagram showing the commutator, limiters, keyers, selectors, such as are utilized in my appa- 'ratus.

' terminals IIII has Turning now to Figure 1, I have shown a signal rectifier I03 having terminals IIJI acros which the signalling currents in the form of A. C. impulses are applied. The input current at the the form MI, as shown in Figure 2a. The output of the signal rectifier I03 thereafter will have the form I43 shown in Figure 2b. The outputs of these signals are fed through the brush I01 to the commutator I05 comprising three segments I23, I25, I21, separated by suitable insulation I29, I3I, I33, respectively. A centering ring H9 is grounded and provides a common terminal point for the three storage condensers H3, I I 5, I II. The rectified impulse is consequently stored-up in the condenser H3 during the time the segnient I23 is under the brush III'I. During the time the charge is being stored by the condenser I I3' the previously stored charge of condenser I15 serves to actuate the recorder IZI ure 2C.

3 through the brush I09, the magnitude of the recording current being proportional ,to the stored charge ,of the rcondenser II5. .Atthesametime, the charge, which had'been stored on condenser II'I previous to the storing of the charge on the condenser H5, is discharged to ground by way of a brush I I I. As the commutator I05 continues .to rotate the condenser H3 comes under the brush I09 to control the magnitude of the 're corder current, while condenser H5 is discharged through the brush I I I and condenser I I'I receives 'a new charge corresponding to the duration of the second pulse received and the operation is cyclically repeated during the transmission of the -picture. The recorder HI will have a recording current whose form is that shown as I45 :i-n Fig- It will be readily appreciated that the use of mechanical commutator-s is not wholly desirablein View of the -fact that --it limits the-speed of operation and, in addition, requires constant attendance in order to avoid improper operation due to accumulation of dirt under the brushes, as well as the difficulties introduced by changes in leakage path of the insulating members. Accordingly, I have provided a thermionic equivalent in Figure 3 of the circuit shown :in Figure 1 for purposes of illustrating the method of pperation of my invention, which equivalent is substantially of any speed limitations and which requires a minimum of supervision in operation.

In Figure 3 I have indicated by means of horizontal dotted lines three channels A, B, C, and

by vertical dotted lines the component parts of each of the channels in order to simplify the understanding of the operation of the circuit. The

put tube. These 'parts are identified onFig. .3 of 40 the drawings between the vertical dotted lines.

The operation of channel C will be described in detail and the mode of operation of channels A and B will be readily understood as each nf'the channels A, B, and C operates in a similar manner.

The input signals are consequently fed to the three push-pull keyers 'VT'I, VTB, and VT?) through the transformers TRI, TR2, .IR3, .as well as to the rectifier-amplifier tube 'VT23 actuating the trigger tube VT25 of a commutator indicated in its entirety by reference character I59 comprising the three tube 'VTI,VT2,"VT3. The tubes 'VTI, VTZ, W3, and VT25 are gas :tubes, and as shown in both the patent and the publication mentioned below, they may, for example, be RC 885-tubes.

The rectifying portion of the tube VT23, which is fed from the input terminals I5I through the transformer I58, functions in the manner of a conventional full wave detector of radio or other carrier signals, such for example as the detector I0 of Fig. 1. The signals applied to the terminals I 5I may be signals I4I of Fig. 2a of .the drawings. These ignals, after rectification, are applied to the grid 16! .of the amplifying portion of VT23. The plate Iii-2 of the'latter is connected to a plate supply source SI by way of the usual load resistor Hi3. A coupling condenser I54 couples the amplified output .of the tube VT23 to the grid I of the tube VT25 which serves as a triger tube for the electronic commutator I60. The control pulses .for the commutator I60 are furnished to the commutator over .a conductor I61.

The 6011 111111313! is of the Shumayd type ,de-

scribed in the Shumard Patent 2,146,862, as well as the article entitled Some Electronic Switch- ,ing Circuits in the May 1938 issue of Electrical Engineering, at page 209 through page 220. Consequently, a detailed explanation of the operation of the commutator circuit will not be given, since these circuits are already well known. It is merely necessary to point out that the operation of the commutator is such that, assuming channel C is operating, the tube VT3 is passing current, while tubes VTZ and VTI are cut off. Upon the arrival of the second impulse tube VT3 is cut oil and VTZis actuated. Upon the arrival of the third impulse tube VTZ is cut oil and VTI is actuated, and the cycle repeats. In this fashion the commutator is synchronized with the incoming signals.

Assuming that a pulse from VTZB has actuated the tube VT3 so that it conducts, a positive pulse from the tube VT3 is applied to the input grid I75 of the limiter tube VT6 over a conductor "I16 and a grid resistor I'TI, the pulse being derived from "the drop across the cathode resistor R9. As a result, plate current in the tube VT6 flows through the resistor RI 6 producing an IR. drop which causes the grid I18 of the second half of the tube VT6 to become negative with respect to its cathode. The negative potential on this grid blocks the flow of plate current in the second half of the tube VT6, reducing the IRdrop across the resistor RZI to zero. This permits the push-pull stage tube 'VT9 to act as an amplifier for the incoming signal, which is applied over conductors ml to the grids I84 and I85 of tube VTQ through the input transformer TR3, since the cut-.ofibias obtained from across the resistor RZI has been removed.

This removal of the out 01f bias occurs since the grid I18 of the second section of the tube VT6 is negative at this stage of the operation with respect .to its cathode as stated above.

The amplified signal is applied to the storage circuit comprising the tube 'VTIZ and the condenser CIZ through the transformer TRG. The. rectifier tube VTIZ permits current to flow in ,only one direction and consequently the condenser CI-Z will begin to charge up. The storage of .the charge continues as long as the signal pulse is applied. It is to be noted that the capacity of the condenser CI2 is made sufficiently large so as to prevent its charging beyond the linear portion of the charging characteristic during the operating cycle. The discharge tube VTI 5 is prevented from operating by the IR .drop across the resistor RIB in channel A which is applied over a channel cross connection I88 and the output tube V'IZI is held inoperative by the plate current drawn by the tube VTI8 through the re: sistor R24 which reduces the screen voltage on the screen grid I89 of the tube VTZI below the point at which plate current can fiow.

The grid I90 of 'the'tube VTI8 is connected to RI 5 in channelB over a channel cross connecting conductor I9I and therefore is positive with respect to the cathode as long as channel B is not operative. As a result, a charge is being stored up in condenser C I2. At the same time the grid I92 of the tube "YTH in channel B is no l ng r biased'by the drop in R2] over a channelcross connection I94 so that plate current will flow if there isany charge on condenser CII .in channel B, and consequently, the charge of .condenser II in channelB will be dissipated or discha g d by the tube VTIII so that the condenser CII will be ready to receive a new charge upon the arrival of the second impulse.

The grid I92 of the discharge tube VTI4 in channel B is connected to the resistor RZI in channel 0 over the channel cross connection I94 and therefore is at zero potential with respect todts cathode when channel C is receiving a triggering or control impulse. At the same time the ouput tube VTI9 of channel A is in an operative condition and will draw a plate current in proportion to the amountofcharge appearing across the condenser CIO of channel A. VTIS operates because the selector tube VTIB of channel A is not drawing plate current due to the fact that its control grid I98 is connected over a cross channel connection 20I to the resistor RI 8 of channel C which applies a negative bias (or a lesspositive bias) to the grid I98 of VTIS of channel A. The screen grid I96 of the output tube V TI9 in channel A is connected to the plates of the tube VTIG which, at times, draw' plate currentlthrough theload resistor R22. When the channel C is cut oil, the flow of plate current "in the tube VTIQ reduces the screen voltage on the screen grid I90 of the tube VTI9 below the point where. plate current can flow. The screen grid 206 of the output tube VT20 in channel B is connectedto the plates of the tube VTI'I. When channel A is cut on, a bias is applied to the grids 201 of the; tubeVTI'I over a channel cross connecting conductor 2 I0. When channel 'A'is cut on, the flowflpiplate current reduces the screen voltage on'the screen grid 206 of the tube V120 belowthe point whereplate current can 'flow. It will be,noted therefore that the fol- 'lowing functions "are performed simultaneously. "The storage circuit in'.channe1 C is receiving a charge from the'incoming signal. The storage circuit in channel B 'is being cleared for a new charge, and theoutput tube of channel A is operating; jdrawing platecurrent in proportion to the circuit of condenser CIO of the storage system in channel A. As soon as the commutator shifts to the nextiposition, which is channel B, the following action takes place. VT9 of channel 0 is rendered inoperative due to the IR drop across the resistor R2I.. The tube VTI I is rendered inoperative by a bias' applied to its grid I92 over the conductor- I94 for the same reason. The tube VTIB is rendered inoperative over the conductor .I9I because of the reduced IR drop across the resistor RI5. The tube V'IZI is permitted to operate because there is no IR drop across the resistor 24 due to the flow of plate current in the tube ,VTIB, and, finally, the tube VTI3 dissipates the. charge across the condenser CID. The tube VTI3 is rendered operative for this purpose over a channel cross connecting conductor 204 which is connected to the resistor R20. Thus, the action continues throughout each .cycle. of operation so that when one channel is receiving a charge the charge of the following channel is being dissipated, while the output tube in the previous channel is operating. The commutator switching is substantially instantaneous (less than 0.1 micro-second) leaving no space or overlap between channels so that the output is substantially continuous and of an amplitude proportional to the signal. It will thus be perceived that the commutator system perates each channel separately, permitting each storage circuit to receive a charge, the

amplitude of which will be proportional to the length of time,a ,signa1 is being passed by the push-pull keyer stage (in CFVD picture signal re- 6 ception this length of time will be governed by the length of the transmitted dot with the commutator synchronized at the dot frequency). After one storage circuit has received its charge, the push-pull keyer in that particular channel is rendered inoperative by the action of the commutator and the next channel is made operative at the same time over one of the channel cross connecting conductors I94, 204, or I88, with the output tube in the first channel becoming operative by virtue f one of the cross-connections 20I, I9I, or 2H! between the commutator limiter of the second channel and the output selector of the first channel.

When the storage circuit of the second channel has received its charge, and the output tube in the first channel has done its work, the push-pull keyer in the second channel is rendered inoperative along with the push-pull keyer tube in the first channel. Control of the push-pull keyer tubes VTI, VTB, and VT9 is provided by the resistors RIB, R20, and R2I in the plate circuits of the tubes VT4, VT5, and VTG as explained above. The output tube in the second channel is permitted to operate by virtue of the fact that connected to the commutator-limiter of the third channel. These cross connections as indicated previously are 2| 0, HI, and 2M. The output tube of the first channel is. rendered inoperative because of the cross-connection between the first channel output selector and the second channel commutator-limiter. The charge stored in the storage circuit of the first-channel is dissipated in a discharge tube in the storage system because the grid of the discharge tube is cross-connected to the output of the commutator-limiter system of the third channel. These cross connections are designated I88, 204, and I94 and are connected to the grids 2 I6, H8, and I92 of the tubes VTI 5, VTI3, and VTI4 respectively. The amount of current appearing in each output tube is dependent upon the magnitude of the charge built up in the storage circuit which, in turn, is dependent upon the time duration of the signal during the charge period. Thus a, complete cycle is set up which continues as long as signals are fed t the input terminals I'5I.

The output current is obtained from the output terminals I53 and they operate any suitable recorder, such as is known in the prior art, which may for example be a light valve or a pyro recorder. It willbe appreciated that other types of tubes may be used without going outside the scope of my invention, and, of course, other modifications of the circuits which I have shown will immediately suggest themselves to those skilled in the art, and therefore it is desired-that the disclosure be construed as covering all forms of converting systems which fallclearly within the spirit and scope of the hereinafter appended claims.

Various alterations and modifications of the present invention may become apparent to those skilled in theart and it is desirable that any and all such modifications and alterations be considered within the purview of the present invention except as limited by the hereinafter appended claims.

Having now described my invention, what I claim is:

1. The method of converting time-modulated signals into amplitude-modulated signals which comprises the steps of receiving time-modulated signals, deriving an electrical charge proportional sipating the derived charge.

3. The method of converting time-modulated signals into amplitude-modulated signals which comprises the steps of receiving time-modulated signals, deriving an electrical charge proportional to the time duration of the signals, utilizing the derived charge to control the amplitude of a uni directional current in proportion to the magnitude of the derived charge, thereafter dissipating the derived charge, simultaneously deriving a second charge from a signal following the first signal and concurrently controlling the amplitude of the unidirectional current by a charge derived from a signal preceding the first signal and cyclically repeating the aforesaid steps.

4. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for receiving timemodulated signals, means for deriving an electrical charge proportional to the time duration of the signals, and means for utilizing the derived charge to control the amplitude of a unidirectional current in proportion to the magnitude of the derived charge.

5. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for receiving timemodulated signals, means for deriving an electrical charge proportional to the time duration of the signals, means for utilizing the derived charge to control the amplitude of a unidirectional current in proportion to the magnitude of the derived charge, and means for thereafter dissipating the derived charge,

6. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for receiving timemodulated signals, means for deriving an electrical charge proportional to the time duration of the signals, means for utilizing the derived charge to control the amplitude of a unidirectional current in proportion to the magnitude of the derived charge, means for thereafter dissipating the derived charge, means for simultaneously deriving a second charge from a signal following the first signal, means for concurrently controlling the amplitude of the unidirectional current by a charge derived from a signal preceding the first signal, and means for cyclically repeating the aforesaid steps.

7. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for supplying timemodulated signals to a plurality of amplifying channels, means for deriving a charge proportional to the time duration of the signals connected in each of the said amplifying channels, means to control the amplitude of a unidirectional current by the said derived charge connected to each of the channels, and commutator means to sequentially and cyclically select each of the control means to control the amplitude of the output current by only one of the said control means.

8. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for supplying timemodulated signals to a plurality of amplifying channels, means for deriving a charge proportional to the time duration of the signals con nected in each of the said amplifying channels, means to control the amplitude of a unidirectional current by the said derived charge connected to each of the channels, commutator means to sequentially and cyclically select each of the control means to control the amplitude of the output current by only one of the said control means, and means to dissipate one of the derived charges.

9. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for receiving timemodulated signals, means for deriving an electrical charge proportional to the time duration of the signals, means for utilizing the derived charge to control the amplitude of a unidirectional current in proportion to the magnitude of the derived charge, means for thereafter dissipating the derived charge, means for simultaneously deriving a second charge from a signal following the first signal, means for concurrently controlling the amplitude of the unidirectional current by a charge derived from a signal preceding the first signal, means for cyclically repeating the aforesaid steps, and means operated by the received signals to control the initiation of the cyclic repetition.

10. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for supplying timemodulated signals to a plurality of amplifying channels, means for deriving a charge proportional to the time duration of the signals connected in each of the said amplifying channels, means to control the amplitude of a unidirectional current by the said derived charge connected to each of the channels, and commutator means synchronized by the supplied signals to sequentially and cyclically select each .of the control means to control the amplitude of the output current by only one of the said control means.

11. An electrical circuit for converting timemodulated signals into amplitude-modulated signals which comprises means for supplying timemodulated signals to a plurality of amplifying channels, means for deriving a charge proportional to the time duration of the signals connected in each of the said amplifying channels, means to control the amplitude of a unidirectional current by the said derived charge connected to each of the channels, commutator means to sequentially and cyclically select each of the control means to control the amplitude of the output current by only one of the said control means, means to dissipate one of the derived charges, and synchronizing means controlled by the supplied signals to regulate the period of said commutator means.

12. An electrical circuit for converting timemodulated signals recurring at predetermined time intervals into amplitude-modulated signals which comprises means for deriving an electrical charge proportional to the time duration of the signals during each time interval of a signal, and means for controlling the amplitude of a unidirectional current in proportion to the magnitude of. the derived charge.

JAMES N. WHITAKER. 

